Reconciliation of experimental results on H2O speciation in rhyolitic glass using in-situ and quenching techniques

In order to resolve the controversy over how to interpret experimental H2O speciation results using in-situ and quenching techniques, we have carried out an infrared spectroscopic study to determine whether the molecular H2O (5230 cm(-1)) and OH (4520 cm(-1)) band intensity variation with measuremen t temperature below glass transition is owing to species interconversion or to the temperature dependence of molar absorptivities. By comparing rhyoli tic glasses with different total H2O content from 0.18 to 0.76 wt.%, we sho w that the peak height of the 4520 cm-l band increases by a similar relativ e amount (about 2% if the baseline is fit with a flexicurve and 10% if the baseline is fit by a straight line) from 25 to 400 degrees C, independent o f the total H2O content. The results show that (1) the molar absorptivities do indeed change with temperature, and (2) in our experiments below the gl ass transition temperature, species concentrations do not change noticeably with temperature, and the band intensity variations are caused mainly by c hanges in the shape of the absorbance bands with temperature. The absence o f unquenchable species reaction in the glass state ton our experimental tim e scale) confirms that speciation data can be obtained using the quench tec hnique from 400 to 600 degrees C. On the other hand, the temperature depend ence of the molar absorptivities must be quantified for the full potential of the in-situ technique to be realised. (C) 1999 Elsevier Science B.V. All rights reserved.